Pigeon sound (3)
I believe the answer is:
coo
'pigeon sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for coo that I've seen before include "Softly say" , "Sound made by doves" , "Dove's noise" , "(Of pigeon) murmur" , "Speak lovingly -- cry softly" .)